This is the story of how I saved some of my light bulbs from extinction by building a useful lab tool with them: an adjustable electronic load.

The Need for Adjustable
Electronic Loads

An adjustable electronic (power) load is a very handy
piece of test equipment in the development of electronics
projects. For example, when you are building a power
supply, there will come a time when you need to
“simulate” a load to see how well your design performs.
To properly test a power supply, an adjustable load is just
the ticket. It allows you to measure how much current the
supply can deliver at a given output and input voltage,
and measure important parameters such as efficiency,
regulation, and ripple under various load conditions.

In the old days, I sometimes would use an
incandescent light bulb as a crude power load when
testing a power supply. Light bulbs were easy to find and
could draw a lot of current — which is actually an
advantage in this application. However, I would often be
limited by the choice of light bulbs available at hand.

Controlling the amount of current drawn from the
supply under test was a trial-and-error affair at best. Then,
it occurred to me: What if I could make a sort of “variable
incandescent DC load?” This would be a very useful tool
for me, and I would use my long abandoned incandescent
lamps ... a win-win situation!

Traditional versus PWM
Adjustable Loads

There are a few different ways to build an adjustable
electronic load. A traditional approach (and one that I
built myself in an earlier project [1]) uses one or more
power MOSFETs in parallel as load element(s). The top
diagram in Figure 2 shows a simplified version of this
traditional arrangement. By adjusting the MOSFET’s gate
voltage (typically with a DC signal), the MOSFET
resistance from Drain to Source changes so you effectively
get an adjustable load (resistance) from the “INPUT”
perspective.

Note that in these types of circuits, the MOSFETs
dissipate most of the power and heat, and thus need to be
fitted with adequate heatsinks. You might even need
cooling fans. (The circuit may also require a power sense
resistor if some sort of feedback loop or measurement is
implemented, but I will stick to an open loop strategy here
for simplicity’s sake.)

The bottom circuit in Figure 2 shows the strategy I
employed instead. The incandescent lamp(s) are placed in
series with a MOSFET. Rather than applying a DC control
circuit to the gate, I applied a variable duty cycle PWM
To illustrate this, I measured the current through this
circuit with and without the series inductor using a small